Method relating to manufacturing of steel

ABSTRACT

The invention concerns a method for the manufacturing of steel in an electric arc furnace, comprising melting of charged steel raw material, substantially iron carrier, characterised in that at least 5 weight-%, preferably at least 10 weight-%, of charged iron carrier consist of granulated pig iron, here denominated GPI.

TECHNICAL FIELD

[0001] The invention concerns a method relating to manufacturing ofsteel in an electric arc furnace, comprising melting charged steel rawmaterials for steel manufacturing.

BACKGROUND OF THE INVENTION

[0002] The dominating steel raw material for manufacturing of steel inelectric arc furnaces is scrap. In year 1994 374.2 millions tons ofsteel were produced in electric arc furnaces, of which 93.5% wereproduced by remelting scrap. Remaining steel raw materials for steelmaking mainly consisted of pig iron and directly reduced iron, hereindenominated DRI.

[0003] DRI is manufactured through a number of different processes,among which Midrex-process is the dominating technique. Other employedtechniques of those, which are known by the denominations HYL, HYLIII,FIOR and FASTMET (trade name). Iron carbide Fe₃C, is another product,which to a limited degree is available as a substitute to scrap.

[0004] Table 1 shows the typical range of composition of DRI along withthe data of Fe₃C.

[0005] The most pronounced advantage of using DRI/Fe₃C materials is thelow content of residuals (Cu, Sn, etc.), which generally are consideredas harmful, which opens for the possibility to dilute scrap of poorquality. The DRI/Fe₃C also is relatively high in carbon that results inCO (g) formation when oxygen is injected into the steel. The CO (g) willreduce the steel nitrogen content and enhance slag foaming. In addition,the consistence of the DRI/Fe₃C composition with time, enables the EAFoperator to have a smooth process with small alterations betweendifferent heats. TABLE 1 Typical chemical composition of DRI and ironcarbide. % DRI Iron Carbide Fe_(tot) 87-94 89.8 Fe_(met) 76-89  1.0 Fe₃C— 90.0 C 0.2-2.4  6.0 S 0.01-0.03 — P 0.007-0.05  — SiO₂ + Al₂O₃ 2.6-6.7 2.4 CaO − MgO 0.2-3.0 —

[0006] The DRI/Fe₃C, however, also causes some negative effects to theElectric Arc Furnace (EAF) process compared to scrap. Relatively highgangue and iron oxide levels result in a higher energy demand. Anestimation shows that each additional percent of oxygen that replaces 1%of iron will cost 49 kWh/ton, which in turn results in increasedelectrode consumption, tap-to-tap time and requires an additional amountof carbon of 6.8 kg/ton. Altogether, producing liquid steel from DRIrequires 240 kWh/ton more than producing the same steel from scrap.

[0007] Consequently, it is evident that the use of DRI/Fe₃C in the EAFis most common where the production of DRI/Fe₃C is cheap, i.e. wherenatural gas is commonly available, usually in combination with lack ofhigh quality scrap and/or production of residual sensitive steel grades,predominantly in several developing countries, where the use of DRI mayrepresent 10-100% of the charged material in some EAF:s.

[0008] A more readily available scrap substitute than DRI/Fe₃C is pigiron. In fact, pig iron is already today charged in many EAF:s, whereinthe pig iron consists of conventional shapes produced in pig ironcasting machine, sand lined pit casing or the like. These pig ironshapes, however, are not designed to fit the requirements of an EAFsteel raw material very well, and particularly it does not promote thecontrol of the melting and decarburisation and reduction processes whichare carried out in the EAF.

BRIEF DESCRIPTION OF DRAWINGS

[0009] In the following description of the invention, reference will bemade to the accompanying drawings, in which

[0010]FIG. 1 in the form of a diagram illustrates how the content ofresidual metals emanating from scrap can be reduced at the manufacturingof steel according to the invention, and

[0011]FIG. 2 and FIG. 3 in the form of diagrams illustrate the effectsof pre-heating and post-combustion, respectively.

DISCLOSURE OF THE INVENTION

[0012] It is the purpose of the invention to provide an improved methodrelating to manufacturing of steel in an EAF (Electric Arc Furnace)comprising melting charged steel raw materials for steel manufacturingand preferably also decarburisation by injection of oxygen gas in themolten metal. The invention is characterised in that x-100 weight-% ofcharged steel raw materials for steel manufacturing consists ofgranulated pig iron, herein denominated GPI. Preferably said GPIsatisfies the following requirements, namely:

[0013] a) that it has a chemical composition containing 0.2-3% Si, 2-5%C, 0.1-6% Mn, the remainder essentially only iron and impurities whichcan normally exist in pig iron produced in the blast furnace process orother shaft furnace process, e.g. in Capola furnace,

[0014] b) that it has a melting point <1350° C., and

[0015] c) that it consists of essentially homogenous particles withsubstantially round or oval shape obtainable through granulation of amelt with the above mentioned composition, comprising disintegration ofa stream of said melt to drops, which are cooled in a water bath to forma granulate.

[0016] For the production of the granulate, a specific method can beused, the general principles of which are described in the U.S. Pat. No.3,888,956. By this known method, raw iron granulate can be produced, inwhich at least 90 weight-% of the granulates consist of particles withshapes varying from substantially round or oval disks to drops andspheres with sizes varying from 1 mm up to 25 mm measured in the largestdimension of the granules. The GPI can be used in this form, butpreferably the fine fraction is removed by screening (this finerfraction can be used as a doping agent in a foaming slag in the EAF, aswill be explained more in detail in the following), so that the GPIwhich in accordance with what is mentioned above is charged to form amelt an/or which is added to a form or remaining melt consists of agranulate which to at least 80 weight-% consists of particles having aparticle size between 2 mm and 25 mm, measured in the largest dimensionof the granules.

[0017] The low area/volume ratio of the round or oval particles of theGPI reduces oxidation during storage and handling, something which hasturned out to be a problem with DRI with its porous structure. Thearea/volume ratio of GPI, however, is higher than of normal pig iron andlarge sized scrap material, and considerably more well defined, whichprovides a better and more reproducible heating and melting features.The round or oval shape of the GPI also results in a relatively highbulk density, approximately 4.5 kg/l, with excellent free-flowcharacteristics. Most commercial scrap grades such as bundles, shredmetal and turnings, have a bulk density of 0.7-1.2 kg/l, table 1. TheGPI's shape also enables easy penetration through the slag layer whenthe iron is injected into the EAF. Finally the GPI, when screened asabove indicated, has a low fraction of fines and is relatively hard,which in combination gives small material losses during handling. TABLE2 Bulk density and residual levels of GPI, DRI and some scrap gradesBulked density (kg/l) % (Cu + Ni + Mo + Sn) Granulated Pig Iron 4.5 0.05(GPI) Direct Reduced Iron  16-2.7 0.05* (DRI) No 1 Bundles. 1.1 0.16 No2 Bundles 0.85-1.0  0.72 No 1 HMS 0.7-0.8 0.50 No 2 EMS 0.65-0.8  1.01Shredded 1.0-1.1 0.56

[0018] According to a preferred embodiment of the invention, there ischarged to the EAF as a steel raw material for making steel, besides GPIalso scrap containing impurities in the form of one or more of themetals which belong to the group of residual metals which consist ofe.g. copper, nickel, molybdenum, and tin. Therein there is achieved anadvantageous dilution of said residual metals in the finished steel meltbecause the GPI contains significant lower levels of residuals (Cu, Sn,Ni, etc.) than scrap, table 1. The dilution effect of GPI addition tothe EAF on the residual content is illustrated in FIG. 1.

[0019] The low residual levels of GPI opens for the possibility of theEAF operator to use poorer scrap quality, FIG. 1. Typically, when usinga mixture of GPI and scrap according to the invention, the addition ofGPI amounts to at least 10%, preferably more than 25%, or even more than40% of the added steel raw materials, the remaining steel raw materialbeing substantially scrap. However, addition of GPI as the sole, 100%steel raw material can be contemplated, particularly when producingsteel intended for flat products for which virgin steel raw material isparticularly advantageous.

[0020] GPI has a higher content of carbon compared to scrap and DRI.When the carbon is decarburised by oxygen injection, the CO (g) purgingreduces the nitrogen level of the steel and chemical heat is generated.Apart from rinsing the steel, the CO formation during oxygen injectionmay be used in order to form a foamy slag. If a carbon injection of 12kg/ton steel is used during normal operation for this matter,approximately 30-40% of the charged material can be substituted by GPIonly in order to balance the carbon injection. An additional benefit ofadding the carbon as GPI instead of injected carbon, is the possibilityof achieving an early boil, i.e. GPI opens for an early slag foaming,which increases the heat efficiency and eliminates any power reductiondue to thermal overload.

[0021] The GPI chemical composition also differs with respect to someadditional properties compared to scrap, DRI and Fe₃C. Thus, GPI has avery low oxide content. DRI/Fe₃C, on the other hand, contains a ratherlarge amount of gangue and unreduced iron oxides, which requireadditional energy to be added. Further, GPI is relatively high insilicon. This silicon is oxidised during melting and oxygen injectionand requires an extra lime addition in order to control the slagcomposition. This lime requires extra energy input in order to heat andmelt the slag former. In the case of GPI, however, the extra energy needis by far compensated for by the chemical heat evolved during siliconoxidation and may even allow the addition of DRI together with GPIwithout need for additional electrical power supply compared to a caseof 100% scrap charge. According to one aspect of the invention thereforethere is added, besides GPI to the EAF, also directly reduced iron, DRI,which contains in weight-% 75-90% metallic iron, 0.2-3% C, 2-7% ganguematerial, mainly SiO₂+Al₂O₃, the balance being substantially iron oxide,FeO, wherein GPI is added at least in an amount such that its content ofsilicon and carbon in combination with the carbon in added DRI willreduce the iron oxide in said DRI to form elementary iron, at the sametime as the oxidation of silicon and carbon in said GPI and DRIgenerates heat to a sufficient degree for compensating the coolingeffect that is caused by the gangue material and the iron oxide in addedDRI and preferably also compensates for the cooling effect because ofadded lime or other basic slag former (Mg- and/or Ca-carrier) forcontrolling the slag composition.

[0022] The GPI can be added through basket charging as well as bycontinuous feeding, e.g. via a vertical scrap chute or by injection.When basket charged, the GPI should be added in the first basket that ischarged in the EAF, wherein a melt is quickly formed because of the lowmelting temperature of the GPI. When adding by injection, the GPIaddition may eliminate at least one basket of scrap. This will decreasethe furnace idle time as well as heat losses. In addition, continuousfeeding of material into the EAF results in a much smoother operation ofthe furnace, compared to batch-wise addition of scrap. At continuousfeeding, foaming slag practice in combination with maximum power inputcan be applied. The high bulk density of GPI also is an advantage whenbasket charging is used only. In conclusion, the possibility ofcontinuously adding steel raw material into the EAF is from a practicalpoint of view a strong argument to use GPI.

[0023] The rather high carbon content of GPI results in a low meltingpoint, that is an early melting is achieved in the furnace. Once aliquid steel pool is present in the furnace, the GPI injection can startthrough the furnace roof. If then a foamy slag is formed on top of thesteel and constant (maximum) electrical power is applied, the evolvedheat from the electric arcs may be balanced by the injection rate ofGPI. This will open for the possibility of keeping the steel temperatureconstant and minimises the thermal gradients in the furnace volume, oneof the drawbacks in a “normal EAF”.

[0024] Temperature control of the steel during melting also increasesthe possibilities of performing refining operations at an early stage inthe furnace and it opens for the possibility of running the EAFsemi-continuously, that is with a rather large hot heel, continuousfeeding of steel raw material and batch-wise bottom tapping.

[0025] If an even more energy efficient production is desired, the CO(g) formed during slag foaming can be subject to post-combustion abovethe steel bath. In addition, the GPI can be preheated by the furnaceexhausts to high temperatures without any risk of environmentallyhazardous emissions, which even more increases the heat efficiency ofthe furnace. These issues are further discussed below.

[0026] In order to illustrate the thermal benefits of using GPI, theenergy needed for melting and heating is calculated on the materialtypes shown in table 3. The calculations are based on the assumptionthat all C and Si are subject to oxidation, the CO formed is notpost-combusted and all formation of SiO₂ during melting is assumed to beneutralised by the addition of CaO or other neutralizing agent. Theenergy required for melting of CaO or corresponding and the energyevolved when CaO, SiO₂ and other oxides are mixing, is assumed to beequal.

[0027] Table 3 also gives the theoretical energy requirement in order tomelt and superheat the materials to a temperature of 1600° C. Givenfigures are per ton produced pure Fe. As can be seen from the table, GPIrequires the lowest amount of electrical energy due to the latentchemical heat available in the material. It can also be understood fromtable 3 that the rather large difference between GPI # 1 and #2 is dueto the difference in % Si; 0.5 and 1.2 respectively. TABLE 3 Materialcompositions and energy requirements for various materials when aimingat different slag basicities. DRI GPI Pure Fe (Midrex) Fe₃C #1 #2Composition % C 0 1.5 0 4.2 4.6 % Si 0 0 0 0.5 1.2 % FeO 0 6 6.6 0 0 %SiO₂ 0 1.5 1.2 0 0 % Al₂O₃ 0 0.8 1.2 0 0 % CaO 0 1.2 0 0 0 % MgO 0 0.3 00 0 % Fe₃C 0 0 90 0 0 % Fe 100 88.7 1 95.3 94.2 [KWh/ton B = 1.2 378 438335 266 217 pure Fe] B = 1.5 378 439 337 268 220 B = 1.8 378 440 338 269223

[0028] The possibility of utilising scrap preheating is illustrated inFIG. 2, which presents a calculation of the energy need at differentdegrees of preheating for the table 3 materials. It should be noticedthat the preheating of scrap is limited to 300° C. due toenvironmentally reasons. Preheating of DRI might also be restricted dueto the pyrophoric behavior.

[0029] If post combustion is carried out in the EAF, the formed CO (g)during decarburisation can be oxidised and the latent chemical heat ofGPI will be even more efficiently used. FIG. 3 shows the theoreticalenergy requirements versus the amount of post combusted CO (g) thatforms CO₂ (g) (100% yield of produced heat). Added material is preheatedto 200° C.

[0030] The invention is particularly suited to be employed for themanufacturing of steel in an EAF (Electric Arc Furnace) comprising theformation of a foaming top slag with a temperature of 1400-1800° C. inthe furnace on top of the surface of the molten metal and supply ofoxygen gas to the molten metal in order to oxidise at least part of theexisting silicon in the melt for heat generation and to oxidise at leastpart of the carbon in the melt for heat generation and to generate gasin the form of CO and/or CO₂ which contributes to the slag foaming, bywhich the supply of oxygen to the melt also brings about oxidation ofmetal elements other than silicon in the melt, in this text generallyreferred to as valuable metal elements, which go into the slag and arereduced there by the addition of reduction agents to the top slag sothat these elements to a considerable degree are recovered to the melt.According to this aspect of the invention, there is, during at least onephase of the production process, a doping agent in the form of aparticle-formed, granulated product is added to the top slag with theaim of creating improved conditions for the reduction of the oxidised,valuable metal elements in the top slag, participating in the reductionprocess itself, contributing to and/or maintaining the slag foaming aswell as adding metal to the melt, said doping agents fulfilling thefollowing requirements, namely:

[0031] a) that it has a chemical composition containing 0-5% Si, 2-7% C,0-3% Mn, the remainder essentially only iron and impurities which cannormally exist in pig iron produced in the blast furnace process orother shaft furnace process,

[0032] b) that it has melting point <1350° C., and

[0033] c) that it consists of essentially homogeneous particles withsubstantially round or oval shape obtainable by granulation of a meltwith above-mentioned composition, comprising disintegration of a streamof said melt to drops, which are cooled in a water bath to form agranulate.

[0034] Preferably the said doping agent is of the same general type thatis used as a steel raw material and which is melted to form a bath ofmolten metal as has been described in the foregoing. Preferably,however, the GPI, which is added as a doping agent to the slag, has asmaller particle size than the GPI that is added as a steel raw materialto the furnace according to the foregoing. More particularly, it isadvantageous to use, as doping agent, GPI which to at least 80 weight-%consists of particles having a particle size varying between 0.5 mm and5.5 mm measured in the largest dimension of the particles. The dopingagent thus may consist of a fine fraction of granulated pig iron, themain part of the granulate having a considerably larger particle size,2-25 mm, being basket charged to the furnace or injected into the meltthat is successively formed. A granulate having said smaller particlesizes has a capacity to penetrate the slag to a desired degree and tokeep themselves suspended in the slag for sufficiently long not only tomelt, which the particles do quite quickly, but also in order that thecontent of carbon and silicon of the granulate shall get sufficient timeto react with the oxides of the valuable metal elements in the slag, andsuccessively agglomerate to form larger agglomerate of molten metal,which sink down through the slag to combine with the melt. The rounderthe particles are, the better they are in terms of their ability topenetrate the slag. In contrast, irregularly shaped ships, flakes, oxidescales, etc. have very poor penetration ability, which is also true forpowder, and cause large losses to the flue system when injected.

[0035] The addition of doping agent can be made through a lance with agas carrier, where the lance can be placed through the slag door,furnace wall or furnace roof, or by mechanical feeding from a positionabove the slag, in the furnace wall or furnace roof. The added dopingparticles melt quickly in the hot slag and form small drops with largeboundary layer area between liquid metal phase and slag, whichkinetically favours reduction of metallic oxides. The doping agentcontains active contents of dissolved carbon and silicon, whichparticipate as melted drops in the reduction reactions. Dissolved carbonforms CO/CO₂-gas, which in turn generates and/or maintains the foamingslag and helps to keep the small metal drops suspended in the slag. Theachieved reduction and foaming implies a number of advantages in theprocess, which in certain cases can be vital for obtaining aneconomically acceptable furnace operation. Thus, the carbon dissolved inthe doping agent has several functions: it contributes to and/ormaintains formation of the foaming slag, it contributes to keeping thesmall molten metal drops suspended in the slag, which maintains thefoaming, and it participates in the reduction processes.

[0036] Also the silicon, which is dissolved in the doping agent, hasseveral functions. Silicon contributes to the reduction of oxidizedvaluable metal elements, which most probably decreases the boundarylayer tension between slag and doping agent, which further acceleratesthe reduction reaction. Furthermore, heat is formed through theoxidation of dissolved carbon and silicon. Oxidation of dissolvedsilicon contributes as well to the formation of slag in the furnace.Finally, the doping agent contributes to a significant addition of ironto the melt when most of the reduction agents—C and Si—in the dopingagent have reacted with the slag and a number of smaller drops haveagglomerated to larger drops which then sink down through the slag layerinto the metal bath.

[0037] In order for the doping agent to be useful as a commercialproduct of high value, with which the electric arc furnace top slag canbe doped to produce the desired result from heat to heat, it isdesirable that the contents of carbon and silicon in the doping agent bekept within relatively narrow limits within the stated outer limits.Thus the contents of carbon and silicon should not vary more than+/−0.5%, preferably not more than +/−0.3% from the assigned target valuewithin said outer limits. Thus the carbon content in the doping agentshould go up to (C_(x)+/−0.5)%, where C_(x) is a number between 3 and4.5. Preferably the carbon content should be (C_(x)+/−0.3)%. In acorresponding way the silicon content should be (Si_(x)+/−0.5)%,preferably (Si_(x)+/−0.3)%, where Si_(x) is a number between 1 and 2.5.The desired contents of carbon and silicon can be obtained throughalloying the raw iron with carbon and silicon after possibledesulphurisation or other treatment of the raw iron.

[0038] The amount of added doping agent can be varied within wide limitsdepending on the composition of the melt, the composition of the dopingagent and other factors. Normally the amount of doping agent added tothe slag according to the invention can go up to between 5 and 60 kg ofdoping agent per ton produced steel, which is added to the slag byinjection into the slag or in another manner to maintain slag foamingand reduction. Simultaneously, oxygen is added in a balanced amount tothe steel to oxidize mainly Si and C in the steel to obtain heat and gasfor the slag foaming. Also other metal elements in the steel, e.g. Feand Cr, are oxidized to a certain degree and then reduced again whenthey reach the slag. Other reduction agents may also be added, besidesthe doping agent according to the invention, e.g. C or Si, to the slagin order to ensure the reduction together with the doping agentaccording to the invention. Preferably, the reduction agent, however,completely consists of the doping agent of the invention, which isadvantageous from several reasons. On one hand the doping agent of theinvention has evident economical merits; on the other hand the processis easier to control if the number of different additions is restricted.

[0039] The GPI which is employed according to the invention, in thefirst place for forming a bath of molten metal, which GPI is basketcharged or is injected in the melt, as well as the GPI which possibly isinjected as a doping agent in connection with foamed slag practice, canbe manufactured according to a number of different methods, comprisinggranulation of a pig iron melt having the above mentioned composition,comprising disintegration of a stream of molten metal to drops, whichare cooled in a water bath to form a granulate. A useful technique,which is known under the trade name GRANSHOT is as mentioned describedin the U.S. Pat. No. 3,888,956, which also describes how the size andshape of the granulate that is being manufactured can be controlledthrough variation of the height of fall of the stream of molten metalbefore the stream is disintegrated to drops and/or of the height of fallof the drops before they hit the water surface in the cooling bath. Inaddition and/or as an alternative the achieved granulate may be a sievedfor the provision of the desired size fraction or desired sizefractions, respectively, according to above.

[0040] Concerning GPI as a charging material, i.e. as a steel rawmaterial—in which above mentioned doping agent is not included—in EAFs,the following applies, which can be utilised according to differentaspects of the invention:

[0041] 1. GPI replaces completely or partly other steel raw materials,such as for example scrap, conventional pig iron, DRI and/or Fe₃C, andcan according to an aspect of the invention be charged through the useof scrap baskets and/or equipment for continuous charging.

[0042] 2. GPI has very good preheating features, especially forpreheating by the flues from the furnace because of the shape andchemical stability of the GPI, which is taken advantage of according toanother aspect of the invention, which characterised in that the GPIthat is charged in the furnace is preheated by the flue (exhaust) gasesof the furnace before charging. This possibility does not exist withscrap, at least not to the same degree, because of the shape of scrapand also because of the risk of formation of dioxine, or with DRIbecause of the pyrophorous character of that material

[0043] 3. GPI has good “free flow” features, which facilitatescontinuous charging, which can be employed according to an other aspectof the invention. Herethrough a plurality of essential advantages can begained, such as

[0044] Less heat losses because of turned-aside furnace roof inconnection with basket charging.

[0045] Shorter power-off time.

[0046] Smoother procedure of process (=higher energy yield) when thearcs continuously and in, a stabile manner hit against a steel bath.

[0047] Possibility of a continuous foamed slag process, i.e. higheryield of supplied energy.

[0048] Possibility to control the temperature of the steel during themelting phase, which means that refining operations may be initiatedduring an early phase.

[0049] Higher productivity because of shorter tap-to-tap times, i.e.shorter times of treatment reduce all time-dependent losses.

[0050] Possibility to run the furnace continuously or semi-continuously,which further increases the productivity of the furnace.

[0051] 4. GPI has a high bulk density, which is an advantage during allphases of the handling of the material, from transportation to charging.

[0052] 5. GPI has low melting point (<1350° C.) which gives an earlymetal melt in the furnace, a possibility which is also utilisedaccording to a number of further aspects of the invention, which aredescribed in the foregoing, in the appending patent claims and/or below.

[0053] 6. The comparatively high contents of C and Si make oxygen gasinjection in the molten metal possible, and hence chemical heat whenCO/CO₂ and SiO₂ are formed, without any greater risk of oxidation ofother alloy elements. The latter is often the case according traditionaltechnique when charged Si does not exist in the steel until after acertain period of time, when oxidation of the alloying elements alreadyhave taken place. Oxygen gas injection also means that a liquid,preferably foaming slag, can be created at an early stage of theprocess.

[0054] 7. If oxygen gas is added to the melt, which is the caseaccording to a preferred embodiment of the invention, the oxidation of Cand Si in GPI means a very low consumption of energy for melting GPI,wherein the heat which is generated through the oxidation of C and Sialso can be used as a contribution to melting any possibly added scrap,and/or DRI and/or for compensating for heat losses because of reductionof iron oxide in any possibly added DRI, addition of lime or other basicslag former for controlling the basicity of the slag, etc.

[0055] 8. Some advantageous chemical features of GPI are, besides thehigh contents of C and Si as above mentioned, also the following:

[0056] Law contents of residuals (Cu, Sn, Zn, Ni, etc.) which accordingto an aspect of the invention can be utilised e.g. for diluting thecontent of such residual elements in the molten metal, when also scrapis used as a steel raw material in the charge.

[0057] Very small or no fraction of oxidic material.

[0058] Homogenous composition; no differences of practical importancewithin the same batch or from batch to batch.

[0059] Particularly advantageous is the invention when GPI is used as acharging material, preferably in the form of a coarse fraction of a pigiron granulate according to above (80 weight-% of the GPI havingparticle sizes between 2 and 25 mm), as well as a doping agent infoaming slag, preferably in the form of a fine fraction of pig irongranulate (80 weight-% having sized between 0.5 and 5.5 mm). When GPI ischarged and is melted in the furnace, a C-and Si-buffer is ensured inthe melt. If the charging of GPI to the melt further is performedcontinuously, the oxidation of alloying elements is further diminished.This creates conditions for valuable metals, in the first place iron andpossibly other existing, oxidised metals to be recovered by reductionthrough addition of only GPI in its function as a doping agent to theslag for securing a complete slag reduction.

1. Method relating to manufacturing of steel in an electric arc furnacecomprising melting charged steel raw material, substantially ironcarrier, characterised in that at least 5 weight-%, preferably at least10 weight-%, of charged iron carrier consists of granulated pig iron,herein denominated GPI.
 2. Method according to claim 1, charecterised inthat said GPI satisfies the following conditions, namely: a) that it hasa chemical composition containing 0.2-3% Si, 2-5% C, 0.1-6% Mn, theremainder essentially only iron and impurities which can normally existin pig iron produced in the blast furnace process or other shaft furnaceprocess, e.g. in Capola furnace, b) that it has a melting point<1350°C., and c) that it consists of essentially homogenous particles withsubstantially round or oval shape obtainable by granulation of a meltwith the above mentioned composition, comprising disintegration of astream of said melt to drops, which are cooled in a water bath to form agranulate.
 3. Method according to claim 2, characterised in that itcomprises decarburisation through oxygen gas injection into molten metalformed in the furnace.
 4. Method according to claim 3, characterised inthat the silicon in said GPI is oxidised at the decarburisation tosilicon dioxide, SiO2, which essentially is collected in a top slag inthe furnace, for the control of the slag composition, wherein there isadded any basic slag former, substantially containing Ca- and/orMg-carriers in such an amount that the slag composition will satisfy therequirement${{2,\quad 8} \leq \frac{{CaO} + {MgO}}{{SiO}_{2}} \leq {3,\quad 6,}}\quad$

preferably the requirement${3,\quad 1} \leq \frac{{CaO} + {MgO}}{{SiO}_{2}} \leq {3,\quad 3}$


5. Method according to any of claims 1-3, characterised in that thesteel is produced batch-wise in the electric arc furnace and that saidGPI is added to the electric arc furnace at an initial stage of thecharging procedure in order quickly to form a pool of molten metal inthe furnace.
 6. Method according to claim 5, characterised in that saidsteel raw material at least partly is basket charged, at least GPI beingadded with the first basket in the charging procedure.
 7. Methodaccording to claim 5 or 6, characterised in that said GPI is injected inthe pool of molten metal which initially is formed or added and/or inthe pool of molten metal that successively is formed in the furnace. 8.Method according to any of claims 1-4, characterised in that the furnaceis operated semi-continuously, i.e. with batch-wise bottom-tapping of aportion, preferably 40-60% of the steel melt, and that GPI is chargedcontinuously or semi-continuously to remaining pool of molten metaland/or to the successively growing pool of molten metal.
 9. Methodaccording to any of claims 1-8, characterised in that said GPI to atleast 80 weight-% consists of particles having a particle size between 2mm and 25 mm measured in the largest dimension of the particles. 10.Method according to any of claims 1-9, characterised in that said GPIhas a bulk density of 3.5-5.5, preferably 4-5 kg/l.
 11. Method accordingto any of claims 1-10, characterised in that said GPI is preheated bythe flue gases from the furnace before charging, preferably that saidGPI is preheated continuously by the flue gases before continuous orsemi-continuous charging.
 12. Method according to an of claims 1-11,characterised in that as a steel raw material there is added to thefurnace, besides said GPI, also scrap which contains impurities in formof one or more of the residual metals belonging to the group of metalsconsisting of Cu, Ni, Mo, Zn and Sn, wherein the addition of said GPIdilutes the content of said residual metals in the steel melt beingformed.
 13. Method according to any of claims 1-11, characterised inthat as a steel raw material there is added to the electric arc furnace,besides said GPI, also directly reduced iron, here denominated DRI,which contains in weight-% 75-90% metallic iron, 0.2-3% C, 2-7% ganguematerial, substantially SiO₂+Al₂O₃, the balance being substantially ironoxide, FeO (iron bound as oxides), wherein GPI is added at least in suchextent that its content of silicon and carbon in combination with carbonin added DRI will reduce the iron oxide of said DRI to metallic iron, atthe same time as the oxidation of Si and C in said GPI generates heat atleast in a sufficient amount to compensate for the cooling action causedby the gangue material and the iron oxide in added DRI.
 14. Methodaccording to claims 12 and 13, characterised in that as a steel rawmaterial there is added to the furnace, besides GPI, also scrapcontaining impurities in the form of one or more of the residual metalsbelonging to the group of metals consisting of Cu, Ni, Mo, Zn and Sn,wherein the addition of said GPI will dilute the content of saidresidual metals in the steel melt that is being formed, wherein alsodirectly reduced iron being added, herein denominated DRI, containing inweight-% 75-90% metallic iron, 0.2-3% C, 2-7% gangue material,substantially SiO₂+Al₂O₃, the balance being substantially iron oxide,FeO (iron bound as oxides), wherein GPI is added at least in such extentthat its content of silicon and carbon in combination with carbon inadded DRI will reduce the iron oxide of said DRI to metallic iron, atthe same time as the oxidation of Si and C in said GPI generates heat atleast in a sufficient amount to compensate for the cooling action causedby the gangue material and the iron oxide in added DRI.
 15. Methodaccording to any of the previous claims, characterised in that 10-20%,preferably 30-50% of the steel raw material consists of said GPI. 16.Method according to claim 1, characterised in that steel raw materialconsists of said GPI to 100%.
 17. Method according to claim 1,comprising the formation of a foaming slag with a temperature of1500-1750° C. in the furnace on top of the surface of the bath of moltenmetal, and the supply of oxygen in the form of oxygen gas to the melt tooxidise at least part of carbon existing in the melt for heat generationand to generate gas in the form of Co and/or Co₂ as a contribution tothe slag foaming, wherein the supply of oxygen to the melt also bringsabout oxidation of other metal elements than silicon in the melt, hereinreferred to as valuable metal elements, which enter the top slag fromwhere they at least to an essential degree are recovered to the meltthrough addition of reduction agents to the top slag, characterised inthat during at least one phase of the one phase of the productionprocess, a doping agent in the form of a particle-formed, granulatedproduct is added to the top slag with the aim of creating improvedconditions for the reduction of the oxidised, valuable metal elements inthe top slag, participating in the reduction process itself,contributing to and/or maintaining the slag foaming as well as addingmetal to the melt, said doping agents fulfilling the followingrequirements, namely: a) that it has a chemical composition containing0-5% Si, 2-7% C, 0-3% Mn, the remainder essentially only iron andimpurities which can normally exist in pig iron produced in the blastfurnace process or other shaft furnace process, b) that it has meltingpoint<1350° C., and c) that it consists of essentially homogeneousparticles with substantially round or oval shape obtainable bygranulation of a melt with above-mentioned composition, comprisingdisintegration of a stream of said melt to drops, which are cooled in awater bath to form a granulate.
 18. Method according to claim 17,characterised in that the particles which are added to the slag consistof particles which to at least 80 weight-% consist of particles having aparticle size varying between 0.5 and 5.5 mm measured in the largestdimension of the particles.